1. Field of the Invention
The present invention relates to a projection display device.
2. Description of the Related Art
With the arrival of a multimedia era, the use of display devices has become popular more and more in every field. Particularly, a projection display device is a notable one because the projection type is more efficient in realizing a large-sized screen than other types. Front projectors are becoming pervasive in presentations, for example, and rear projectors are becoming pervasive as home theaters.
Recently, projectors of the type utilizing, instead of CRT projection, a liquid crystal panel or a DMD (Digital Mirror Device, see, e.g., Japanese Patent Laid-Open No. 10-78550), which modulates the amount of light by changing a mirror angle, have come into widespread use because they are suitable for high luminance and high definition.
However, those projection display devices are inferior in image quality to commonly used direct-view-type CRTs. For this reason, when users desire high-quality display (i.e., display representing an image with a greater depth in color quality), direct-view-type CRTs are employed in many cases even though the screen size is comparatively small. The term xe2x80x9chigh image quality (depth in image quality)xe2x80x9d used herein means that a display device has a large dynamic range (i.e., the device is able to display an image with high contrast and many-levels of gradation). Because of an ability of modulating the luminance in accordance with, e.g., the intensity of an electron beam, a CRT can realize a dynamic range of about 1000:1, for example, when only a particular area (partial area) is displayed in white. Accordingly, the CRT has the potential to display a white image whiter and a black image blacker, and hence realizes excellent image quality. The CRT, however, has a problem that an available screen size is about 40 inches at maximum due to, e.g., a limit in tube size and a greater screen size is difficult to achieve from the technical point of view.
On the other hand, when applied to projection display devices, the CRT type is disadvantageous in the engine size, brightness, and definition. For this reason, as mentioned above, the liquid crystal type and the DMD type suitable for high luminance and high definition have become prevalent in recent years. In those types of display devices, a liquid crystal device or a DMD serves as an optical modulator for modulating a beam of light. A light beam from a lamp is illuminated to the liquid crystal device or the DMD, and a light image is projected onto a screen in an enlarged size through a projection optical system. Therefore, a dynamic range of the projection display device is primarily determined by the dynamic range specific to the liquid crystal device or the DMD.
A practical dynamic range is about 300-400:1 for the liquid crystal device and about 500-600:1 for the DMD. Thus, display devices of the liquid crystal type and the DMD type have a problem that they cannot surpass a display device of the CRT type in only one point of high image quality (large dynamic range).
Accordingly, it is an object of the present invention to provide a system capable of realizing a projection display device which has high image quality (large dynamic range) in addition to its own specific features, i.e., a large-sized screen and high definition.
Furthermore, the system is inexpensive and practical because the above object is achieved by the system based on a combination with an existing device in the state of the art, such as a liquid crystal device and a DMD.
To achieve the above object, according to a projection display device of the present invention, between a light source and an optical modulator (liquid crystal device or DMD), there is provided a unit for adjusting an amount of light illuminated to the optical modulator (illumination-light amount modulating unit). Also, a projection display system of the present invention comprises the projection display device, a circuit for processing a signal applied to the optical modulator in accordance with the amount of illumination light, and a unit for writing the signal in the optical modulator.
With the present invention, since the illumination-light amount modulating unit is provided between the light source and the optical modulator, it is possible to illuminate a dark screen image with a reduced amount of light and a bright screen image with an increased amount of light. Consequently, a larger dynamic range than in the case of illuminating the optical modulator with a constant amount of light can be realized.